Does a Mordant Make a Natural Dye Brighter or Longer Lasting?

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Talk It Over

Although most of the dyes (colors) in your clothes are human-made today, people enturies ago used plants to color their clothes. They also tested substances, called mordants, that they thought made the colors brighter or longer lasting. How can you test a mordant to see whether they were right?


  • Water
  • Measuring cup
  • Small saucepan
  • Set of measuring spoons
  • Alum*
  • Cream of tartar*
  • Spoon for stirring
  • 4 10-cm-x-10-cm (4"-x-4") squares of white, cotton fabric (perhaps cut from an old T-shirt)
  • 2 safety pins
  • Stove
  • The outer skin from a yellow onion


  1. Into the small saucepan, measure 2 cups of water, 2 teaspoons of alum and ½ teaspoon of cream of tartar. Stir with the spoon to dissolve. This is the mordant you will test.
  2. Pin a safety pin into each of 2 pieces of your cloth. Put these pieces in the saucepan. Set the other 2 pieces (without pins) aside.
  3. Ask an adult to put the saucepan on the stove for you. Heat the water and the cloth to boiling. Reduce the heat and simmer for 10 minutes.
  4. Ask the adult to discard the hot liquid for you. Let the fabric pieces dry overnight.
  5. You now have four dry pieces of fabric. The two that have been treated with the mordant have pins in them.
  6. Into a clean saucepan, measure 2 cups of water. Add the onion skins and all four pieces of fabric. Repeat steps 3 and 4.
  7. When the pieces are dry, compare the colors. Did the mordant make a difference in the color?
  8. Set one mordant-treated piece and one untreated piece aside. Wash the other two pieces in a washing machine several times. Or leave them in a strong light for a few days. What happens to their color? How does it compare with the freshly dyed pieces?

Stay Safe

Do not attempt this experiment without adult help. The stove is hot, and boiling liquids can cause serious burns!

Go Easy

Skip testing the mordant. Simply experiment to see what colors you and your adult helper can get from different natural plant materials. Try dying white fabric with roots, bark, leaves, berries, twigs, fruit skins and pits, and more.

Go Far

You can use the "Go" procedure to test a variety of other mordants, including vinegar and household ammonia. Your science teacher may be able to provide you with small amounts of potassium dichromate,iron (II) sulfate, tin (II) chloride, or copper (II) sulfate, which are also sometimes used as mordants. Try to design and carry out an experiment to test the question, How do different mordants interact with different natural dyes to affect how long color lasts after repeated washings (or after exposure to sunlight)?

Show Your Results

For "Go Easy," display your dyed fabric squares along with the plant materials you used to produce them.

For "Go," display your squares showing how the mordant affected the original color as well as the color after washing or exposure to sunlight. Write a paragraph that describes any differences and explains why you think they happened.

For "Go Far," make a table that shows the pieces you dyed and the mordants you tested.You might put them in a table like this:

Dye Mordant
None Alum and Cream of Tartar Vinegar Copper (II) Sulfate
Dandelion leaves Put fabric samples in these squares      
Grape skins . . .and so on        

Tips and Tricks

To compare the different mordants and treatments (washing or fading) most easily, put your fabric pieces side-by-side under a bright light and study the colors carefully. You will see differences that you an describe.

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